The small fire started when a piling was being driven into the ground as part of the Interstate 794 project near Clybourn Avenue and Broadway Street. A geosciences expert WISN 12 News spoke with said methane can be easily trapped underneath the surface.

"I'm guessing near the Public Market, it's less than 100 feet of till," said Patricia Burke, the geology collections manager at the Milwaukee Public Museum.

The museum has an exhibit dedicated to Milwaukee's deep tunnel, which was a project on which she worked on.

Burke said she was watching closely when a construction crew working on the I-794 project discovered a ball of flames while driving pilings Tuesday.

Crews alerted 911, the project was halted and the Public Market was evacuated for the day.

The Department of Transportation, in charge of the project, told WISN 12 News the contractor handled everything properly.

Even with all the planning, the DOT said situations like this are unavoidable. That is a theory supported by the geoscientist.

"If you peeled off all the soil, this is where you would be -- at the surface of the bedrock," Burke said.

Under the surface, loose rock can house pockets of gas, largely because Milwaukee is built on a swamp.

"Swampy ground has organic material that's decomposing, and as it decomposes you get methane gas buildup," Burke said.

It can be worrisome because it's nearly impossible to detect and doesn't take much to create a fire.